Secondary battery protection circuit

1. A secondary battery protection circuit for protecting a secondary battery by controlling a metal-oxide-semiconductor transistor inserted in a power supply path between the secondary battery and a load, the secondary battery protection circuit comprising: a first terminal connected to the power supply path at a position between the secondary battery and the metal-oxide-semiconductor transistor; a second terminal connected to the power supply path at a position between the load and the metal-oxide-semiconductor transistor; a third terminal connected to a gate of the metal-oxide-semiconductor transistor; a fourth terminal connected to a back gate of the metal-oxide-semiconductor transistor; an anomaly detection circuit configured to detect an abnormal state of the secondary battery; a control circuit configured to output a switch control signal based on the detected abnormal state of the secondary battery; and a switch control circuit configured to cause a gate control signal for controlling the gate of the metal-oxide-semiconductor transistor to be output from the third terminal, and cause a back gate control signal for controlling a voltage of the back gate of the metal-oxide-semiconductor transistor to be output from the fourth terminal, based on the switch control signal output by the control circuit, wherein the switch control circuit includes a first switch configured to connect the fourth terminal with the first terminal and a second switch configured to connect the fourth terminal with the second terminal; the control circuit is configured to output the switch control signal to turn on the first switch in response to detection of a first type of the abnormal state, and to output the switch control signal to turn on the second switch in response to detection of a second type of the abnormal state; and at least one of a resistance value between the fourth terminal and the first terminal connected via the first switch and a resistance value between the fourth terminal and the second terminal connected via the second switch is greater than an on resistance value of the metal-oxide-semiconductor transistor.

2. The secondary battery protection circuit according to claim 1 , wherein the control circuit is configured to output the switch control signal to turn on the first switch in response to detection of an abnormal discharging state of the secondary battery, and to output the switch control signal to turn on the second switch in response to detection of an abnormal charging state of the secondary battery.

3. The secondary battery protection circuit according to claim 1 , further comprising: a discharging circuit configured to discharge electric charges of the secondary battery from the fourth terminal in response to detection of overcharging of the secondary battery.

4. The secondary battery protection circuit according to claim 1 , further comprising: a first resistor disposed in an electric current path between the fourth terminal and the first terminal; and a second resistor disposed in an electric current path between the fourth terminal and the second terminal.

5. The secondary battery protection circuit according to claim 1 , wherein the secondary battery protection circuit is implemented as an integrated circuit.